The invention relates in general to direct tension indicating washers and in particular to direct tension indicating washers that self indicate when the proper bolt tension has been achieved.
High strength metal bolts, along with their corresponding nuts, when used in metal connections, are almost always intended to be installed so that they are "tensioned." The "tensioning" is usually accomplished by turning the nut relative to the bolt a specified amount of rotation or until a certain torque resistance has been reached. The tension in the bolt stretches the bolt and compresses the connection plates between the bolt head and nut so the connection plates do not slip relative to each other. It is this slip resistance which holds structures such as bridges and buildings together.
FIG. 1 is a top view of a conventional direct tension indicating washer 10, commonly referred to as a DTI. The direct tension indicating washer 10 includes a series of protuberances 12 formed on a first surface 14. As shown in FIG. 2, corresponding indentations 16 are formed in a second surface 18 of the direct tension indicating washer 10. As shown in FIGS. 3 and 4, the direct tension indicating washer 10 is manufactured through a stamping operation in which the blank metal washer 10 is placed between a tool 20 and a die 22. Pressure is applied to the tool 20 (e.g. by a press) and a protrusion 24 on the tool 20 creates the protuberance 12, and the corresponding indentation 16, in the direct tension indicating washer 10 as shown in FIG. 4.
The DTI is manufactured so that when the tightening of the bolt forces the protrusions 12 to compress, or reduce height, sufficiently, the bolt is said to have sufficient tension to meet code requirements. Determination of "sufficient" protrusion compression is accomplished in the field by the attempt to insert a feeler gage of specified thickness (e.g. 0.015") into the residual gap between the underside of the bolt head and the first surface 14 of the DTI. FIG. 1A shows a feeler gage being placed on a first surface of a direct tension indicating washer to test bolt tension (the bolt is not shown for clarity). If the attempt to insert the feeler gage is unsuccessful, that is, the inspector cannot insert the feeler gage into the gap half way radially around the DTI circumference between the DTI protuberances, the residual DTI gap (the space between the first surface of the direct tension indicating washer and the bottom of the bolt head) is said to be, on average, of dimension less than the thickness of the feeler gage, and is therefore judged acceptable. Conventional direct tension indicating washers are manufactured so that surfaces 14 and 18 are parallel and perpendicular to the longitudinal axis of the bolt which they are made to fit on, that is when the bolt is inserted through the inner diameter.
Conventional direct tension indicating washers, though well suited for their intended purposes, have several drawbacks. First, the process of attempting to insert the feeler gage into the DTI gap is time consuming and must be done after the bolt installer has stopped the tightening process. Second, over time, bolt tension declines as the bolt/nut threads seat into the mating steel surfaces and as external loads shake the connections and try to pry the bolt/nut apart. This problem is especially prominent in connections that are subject to vibration such as bridges, structures subject to earthquakes, connections supporting moving cranes and other equipment, etc. As the bolt tension declines, so does the clamping force and the capacity to carry external loads by resistance to slip. The conventional direct tension indicating washer 10 is flat and has the disadvantage that, after bolt installation, the protrusions 12 are essentially flattened inelastically and will not spring back to their original positions if the bolt begins to loose tension.
In addition, the conventional direct tension indicating washer, when manufactured in relatively small sizes (e.g. for a 1/4 inch diameter bolt), does not function reliably because of the difficulty in accurately using a feeler gauge with such small washers. Also, the relatively small direct tension indicating washers cannot accurately indicate low bolt tensions (typically used with small bolts) due to the process of fabricating the protuberances. Accordingly, there is a need in the art for improved direct tension indicating washers.